Conventionally, in the preparation of medicinal tablets with use of a rotary powder compression molding machine of this type, if the material powder for such tablets consists only of prescribed drug ingredients, there may arise troubles including the so-called sticking such that the material powder or a tablet sticks to a punch or a die. A conventional method that has been generally employed to obviate such troubles includes mixing a powder lubricant, such as magnesium stearate, with prescribed drug ingredients to prepare material powder for tablets, and compressing the material powder into tablets, because this method is easy in the preparation of tablets.
With increasing attention focused on the geriatric care in recent years, there are increasing demands for a tablet of the type which can be dissolved easily in the oral cavity for aged persons or like persons to swallow it easily and for a tablet of the type which can be dissolved immediately after deglutition to exhibit its drug efficacy. However, such a tablet prepared by the aforementioned conventional preparation method has a difficulty in responding to such demands because the powder lubricant mixed therein acts to inhibit disintegration and dissolution of the tablet. There exists another problem that the powder lubricant mixed in the tablet makes the tablet easy to crack.
In view of the purpose of the powder lubricant to prevent the sticking, the powder lubricant need not be mixed with the prescribed drug ingredients. It must be quite possible that material powder consisting only of the prescribed drug ingredients is used if the powder lubricant is attached to a portion where the sticking is likely, such as a punch surface. Apparatus developed with attention focused on this point include one which is configured to spray and coat upper and lower punches and die bore with the powder lubricant prior to compression, and one which is configured to compress only the powder lubricant as a dummy prior to compression of intended tablets thereby coating the upper and lower punches and the die bore with the powder lubricant.
With such apparatus, however, the so-called contamination problem occurs such that the powder lubricant scatters around during spray coating and is then mixed into the prescribed drug ingredients, or, to the contrary, the prescribed drug ingredients are mixed into the powder lubricant during spraying. In addition, in some cases, the powder lubricant is attached to a punch and the like non-uniformly. Further, the latter apparatus calls for a compression mechanism for compressing the powder lubricant, thus raising problems that the apparatus is enlarged in size and that the compression speed lowers to about ½ of a typical compression speed.
Among such apparatus of the type configured to coat required portions with the powder lubricant as described above, there is an apparatus configured to charge the powder lubricant electrostatically prior to spray coating and then coat a mold for powder molding with the powder lubricant thus electrostatically charged, as described in patent document 1 (Japanese Patent Laid-Open Publication No. 2002-327204) for example. The invention described in patent document 1 is configured to electrostatically charge the powder lubricant by friction using a charger gun and then jet the powder lubricant.
In the case of the powder lubricant electrostatically charged, the charged condition of the powder lubricant is difficult to control because the powder lubricant is electrostatically charged by friction according to the invention described in the aforementioned patent document, though static electricity of the powder lubricant causes the powder lubricant to attach to the end faces of respective of the upper and lower punches and to the die bore of the die reliably. For this reason, equal amounts of the powder lubricant are attached to respective of the end face of the upper punch, the end face of the lower punch and the die bore of the die in spite of the fact that these end faces and die bore have different areas to be attached with the powder lubricant. This means that the amount of the powder lubricant attached to each of the lower punch and the die bore is insufficient because the lower punch and the die bore have a larger area to be covered with the powder lubricant than the upper punch.